This invention relates to a process for producing a reducing gas which can be used for reducing ores. The reducing gas contains carbon monoxide and hydrogen and low amounts of carbon dioxide and water vapor.
In the iron- and steel-producing industries, it has often been difficult for years to provide adequate quantities of coke in the qualities required for the operation of blast furnaces. The bottlenecks to be overcome may be due to an inadequate coking plant capacity or to a lack of cokable coal.
Methods enabling a decrease of the consumption of coke in the blast furnace have been known for years. These methods comprise, e.g., an increase of the temperature and/or oxygen content of the blast furnace blast, the blowing of fuel oil into the blast furnace, or the increase of the pressure in the lower part of the blast furnace. More recently, direct reducing processes have been developed with the object substantially to avoid a consumption of coke in the reduction of ore and to reduce the ore mainly or exclusively by reducing gases.
Such reducing gas should have high CO and H.sub.2 contents whereas its contents of water vapor and CO.sub.2 should be minimized and its methane content should be low. Finally, such reducing gas should be as hot as possible.
It is known to convert gaseous and even liquid hydrocarbons, even of fuel oil grade, into a gas having high CO and H.sub.2 contents by a gasification with the aid of pure oxygen. The non-catalytic, thermal process is exothermic so that sulfur-containing fuel may be used. The method has the disadvantage that an excess of oxygen must be used in order to attain the high temperatures required for complete cracking of the feed hydrocarbons. Because of the surplus of oxygen, the product gas has a higher CO.sub.2 content, which is detrimental in a reducing gas.
It is known to effect this partial oxygenation of hydrocarbons in two stages and to carry out a catalytic process in the second stage, if desired. DAS 1,226,545 describes a process of producing a gas which is made from hydrocarbons and which is suitable as a reducing gas and contains mainly CO and H.sub.2, whereas it has a low CO.sub.2 content. In that process the C : O ratio in the mixed feedstocks consisting of hydrocarbons and oxygen and/or air is adjusted almost to 1 and the reaction is carried out in two stages. The first reaction stage is carried out in a swirl chamber, to which at least one component is admitted at high speed. The second stage is carried out in a catalyst-free, hot-blast stove (cowper stove), which is heated to about 1300.degree.C.
It is also known to produce gases having high CO and H.sub.2 contents by cracking hydrocarbons with water vapor in contact with indirectly heated catalysts containing nickel on a refractory carrier material at temperatures above 750.degree.C. This process is widely used to produce synthesis gas and has the advantage of requiring no pure oxygen. On the other hand, it depends on sulfur-free feedstocks and is restricted to the processing of gaseous or readily evaporable liquid hydrocarbons of the gasoline and naphtha ranges.
When used for the production of a reducing gas, this process has the further disadvantage that the hydrocarbons must be reacted in the presence of an excess of water vapor in order to suppress the Boudouard reaction, which would result in a formation of elementary carbon, which is deposited on the catalyst and may finally clog the catalyst layer and the pipelines.
The low contents of CO.sub.2 and CH.sub.4 desired in a reducing gas can be attained in this process by the use of high final cracking temperatures, which may be above 1000.degree.C. The most important difficulties arising in a process where a reducing gas is produced by reforming hydrocarbons with steam involve a decrease of the amount of water vapor required for the reaction and the suppression of the formation of carbon black.
These difficulties may be avoided by some measures which have been used in practice.
The hydrocarbon and water vapor feedstocks may be preheated to higher temperatures individually or in a mixture. Preheating temperatures of 400.degree.- 450.degree.C. are used in the production of synthesis gases. A higher preheating up to about 600.degree.C. can be carried out only in a heater made from special high-alloy material, which are otherwise required only in the tubular reactor itself.
In another process, part of the water vapor required for the reaction is replaced by carbon dioxide, which is recovered from the product gas by scrubbing and is returned to the reforming process carried out with steam. In this case too, a preheating to high temperatures, up to about 800.degree.C., is required. It is also known to use catalysts having different activities in the tubular heater in such a manner that the tubes contain a low-activity catalyst at their inlet end so that at that end the heating of the reaction mixture predominates over the actual reaction. In this way, the development of dangerously high CO concentrations is avoided particularly at the beginning of the reaction. Depending on the composition of the catalyst and particularly of its support, slight amounts of carbon black may be deposited in the inlet zones of the reactor tubes but these are not sufficient to clog the flow paths.
At its inlet end, the catalyst layer may comprise alkali-containing catalysts, which are effective at extremely low steam-carbon ratios. Although carbon black is deposited on such catalysts, the deposited carbon is consumed by accompanying reactions so that carbon black is not deposited in large amounts. These nickel-containing catalysts which comprise a support and which in most cases are alkalinized by an addition of potassium carbonate are sensitive to elevated temperatures. Their alkali content is volatile at temperatures much in excess of 850.degree.C. As the operation proceeds, the alkali content of the catalyst is progressively lost so that its activity changes constantly and the formation of carbon black is increasingly promoted. The volatilized alkali compounds are deposited on colder parts of the plant, where they give rise to clogging and corrosion. Catalysts of this kind are described for example in USP-Specification No. 3 417 029.
Even in combination, these known measures do not enable a reaction of hydrocarbons and water vapor under conditions which are sufficiently remote from the carbon black limit and with formation of a product gas in which the water vapor content is minimized and which can be directly used as a hot reducing gas without having to be cooled substantially below the temperature at which it is formed.